1. Field of the Invention
This invention relates to an improved anionic dispersion polymerization process for the preparation of linear block copolymers containing A) blocks of polymerized styrene or alkyl-substituted styrene units, which are the end or terminal block of the polymer, and which alternate with B) blocks of polymerized conjugated diene units. The invention also relates to improved block copolymers of the foregoing type which have a highly ordered lamellar microstructure.
2. Information Disclosure Statement Including Description of Related Art
The following information is disclosed in accordance with the terms of 37 CFR 1.56, 1.97 and 1.98.
Processes for producing linear multiblock copolymers by anionic dispersion polymerization are known in the art. Thus, U.S. Pat. No. 3,770,712, issued Nov. 6, 1973 to Schwab, discloses this type of process used to produce diblock copolymers containing a polystyrene block and a second block which may be polyisoprene or polybutadiene; U.S. Pat. No. 4,871,414, issued Oct. 3, 1989, to Gunesin et al., teaches the use of the same type of process to produce block copolymers containing at least five blocks wherein blocks of a polymer of styrene or an alkyl-substituted styrene are the terminal blocks and alternate with blocks of a polymer of a conjugated diene; and U.S. Pat. No. 4,985,505, issued Jan. 15, 1991 to Gunesin et al. teaches the preparation of block copolymers of a styrenic compound and a conjugated diene by anionic dispersion polymerization utilizing propane or n-butane as a dispersion medium. The polymers of the latter two patents are described as having good or excellent physical properties such as elongation and impact strength, and as being suitable for blending with crystal polystyrene to improve its physical properties. Although the anionic dispersion polymerization processes disclosed in these patents can be operated at relatively high conversion rates to produce products having good mechanical properties, further improvements in conversion rates and product properties would be very beneficial.
It is also known in the art to utilize anionic solution polymerization to produce block copolymers comprising blocks of polymers of a vinylaromatic hydrocarbon, e.g., styrene, with blocks of a polymer of a conjugated diene, e.g., butadiene or isoprene. Thus, U.S. Reissue Pat. No. 27,145, issued Jun. 22, 1971 to Jones, and U.S. Pat. Nos. 4,835,220, issued May 30, 1989 and 4,891,410 issued Jan. 2, 1990, both to Bronstert et al., disclose the use of anionic solution polymerization to produce triblock copolymers wherein the terminal blocks are polymers of a vinylaromatic compound, e.g., styrene, and the central block is a polymer of a conjugated diene, e.g., butadiene. Bronstert et al. also teach that a mixture of a vinylaromatic compound and a conjugated diene may be added to a previously formed living polymer of vinylaromatic compound (A block), in which case the conjugated diene initially tends to be the monomer being predominantly polymerized, with most of the vinylaromatic compound in the mixture being polymerized after a major part of the conjugated diene has been polymerized to form a B block. This results in the formation of an A-B-A type of block copolymer in which the B block and second A block are separated by a transitional portion containing an increasing amount of polymerized vinylaromatic compound before evolving into the second A block. Anionic solution polymerization processes such as those described in these references, when compared with anionic dispersion polymerization processes, have the disadvantages that it may be difficult to obtain a sufficiently high molecular weight of the polymer due to the fact that solutions of higher molecular weight polymers have viscosities much higher than those of lower molecular weight polymers, making the former solutions much more difficult to treat and handle, and that considerably more energy is generally required to separate and purify a polymer in solution than one that is in the form of a dispersion.
None of the dispersion or solution polymerization processes disclosed in the foregoing patents includes the initial addition to the reactor zone of all the vinylaromatic compound polymerized to form the final block copolymer before any conjugated diene is added. This feature of the process covered herein, as more fully described hereinafter, results in higher conversion rates and improved mechanical properties of the polymer, as compared with processes wherein the vinylaromatic compound is added sequentially as is conventional in the art.